This invention refines the surfaces of metal articles for cosmetic purposes and/or for mechanical functioning purposes, so that the surface of those articles is isotropic, superfinished, and of specular brightness. This invention encompasses both a method for refining these surfaces, and the novel articles that result from the practice of that method. The improved surfaces produced by this invention may yield improved performance in the parts processed.
There are a variety of metal articles for which machining/grind lines are a problem. Examples of mechanical parts with critical working surfaces include splines, crankshafts, camshafts, bearings, gears, couplings, and journals. For these parts, poor surface contact performance caused by lines can increase friction, torque, noise, vibration, operating temperature, and impair lubricity, and negatively impact failure in areas of wear, scuffing, plastic deformation, and contact fatigue and/or bending fatigue. For gears or other parts placed in a demanding environment such as the drivetrain of a helicopter or racing car, resistance to these types of failures in effect defines the useful life of the article.
Critical surfaces (including recessed areas) have conventionally been refined through various machine grinding/polishing processes. But those processes have multiple drawbacks. For complex shapes, machine grinding tools are very expensive, require skilled operators, and undergo excessive wear. Metal parts having an HRC of approximately 42 and higher are not well suited for these techniques. Machine grinding often leads to directional grind lines, and can damage the heat treatment of a metal surface, creating potential failure sites. Finally, machine grinding is carried out on a part-by-part basis, and as such, is plagued with problems of repeatability and uniformity.
REM Chemicals, Inc. has developed and described in its patents techniques that refine metal parts, on a mass process basis, to a smooth and shiny surface. Those techniques have been used commercially for many years in which the process objective is directed primarily to the cosmetic appearance of the part rather than to its mechanical performance. To that end, REM's U.S. Pat. No. 4,491,500 discloses an improvement to traditional mass finishing methods, in which certain chemicals are added to a mass finishing device (such as a vibratory bowl or tumbling barrel) in combination with ceramic bodies (called “media”) and one or more metal workpieces. The chemicals are mildly reactive to the metal, creating a soft coating (called “blackmode”) on the surface, which is removed through vibratory agitation with the media. The resulting surface is smooth and shiny. The media employed in the '500 patent are abrasive—i.e., they are (compared to mildly or non-abrasive media) more rapidly degraded during the finishing process.
REM's U.S. Pat. No. 4,818,333 discloses an improvement to the process of the '500 patent. That patent describes the use with chemicals of ceramic media having a density of at least 2.75 g/cc, and which are comparatively free of abrasive grit, as is commonly found in vibratory finishing media. Suitable media identified in that patent include ceramics of silica and alumina, in combination with other metal oxides. The claims of that patent characterize that media based upon the percentage of weight loss when employed in a vibratory finishing bowl under certain, specified conditions.
Neither REM patent identifies any improvement in the mechanical performance of articles finished using the disclosed processes. Nevertheless, REM has demonstrated that gears, bearings, and other articles processed in accordance with the '500 and '333 patents can enjoy a significant enhancement in performance. And REM has used the processes of those patents commercially for that purpose. For example, U.S. Pat. No. 5,503,481 describes the use of the '333 patent process to give an isotropic surface on bearings, thereby imparting a greater fatigue life for those parts. However, the media employed in the '500 and '333 patents are not ideally suited for finishing processes aimed at enhanced mechanical performance. The '333 patent media have an average diamond pyramid hardness (DPH) value of at least 890, and therefore impart a mechanical texture to part surfaces that are exposed to it. Though the present invention is still applicable to cosmetic finishing, this invention addresses the problem of media hardness by using media (such as metals and/or plastic) that are softer, yet non-abrasive.
Softer non-abrasive media have been used commercially for the refinement of metal surfaces in the past. The ABRIL process, for example, has employed zinc media, but in combination with an abrasive compound.
REM has, more than a year before the filing of the present application, made commercial use of certain plastic abrasive media in combination with reactive chemicals to finish brass and stainless steel parts. But those processes produced surfaces with an Ra (6–10 microinches) that was insufficient for specular brightness or superfinishing. Through the processes disclosed herein, REM has been able to superfinish metal articles to a superior isotropic surface.